While you could use Linq (Select with Index parameter), in this case simple enumeration could be faster:
List<int> list = new List<int>();
for (int i = 0; i < list.Count; i++)
{
if( list[i] == 8) ndxmatches.Add(i);
}
Dim list As List(Of Integer) = New List(Of Integer)()
For i As Integer = 0 To list.Count - 1
If list(i) = 8 Then ndxmatches.Add(i)
Next